Long ago radio transceivers universally operated with single antennas because the wavelengths involved were large and multiple antennas in an array were impractical. Now at the very high frequencies that are common in cellular telephone transceivers, each antenna can be quite small and arrays of antennas can be used to great advantage. Also, signal processing technology is readily available to take advantage of performance improvements possible with multiple antenna systems. This is especially true of cellular telephone base stations that need to improve the received signal strengths and/or steer the transmitted signals in particular directions to improve performance.
Systems with an array of multiple-antenna array are therefore being proposed and implemented. In some cases, the simple difference in location of one antenna over another in the same antenna array will result in a stronger, or weaker, received signal. Such spatial "diversity" is now conventionally used to improve radio system performance and reliability. The difference in received signal strengths amongst individual antennas in an otherwise seemingly closely grouped array may result from the way reflected multipath signals arrive at each one. Smart antenna systems also are being proposed which add adaptive "spatial" processing to improve performance. Each antenna contributes to a weighted combination of received signals and the combination can then be input to a spatial processor which determines optimal weights for improved performance. This can even allow for more than one "spatial" channel to exist in a conventional (frequency, time-slot, and/or code) channel. Such multiplexing is sometimes called spatial division multiple access (SDMA). For a description of smart antenna SDMA systems that can work with more than one spatial channel per conventional channel, see, for example, co-owned U.S. Pat. Nos. 5,515,378 (issued May 7, 1996) and 5,642,353 (issued Jun. 24, 1997) entitled SPATIAL DIVISION MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEMS, Roy, III, et al., inventors, and co-owned U.S. Pat. No. 5,592,490 (issued Jan. 7, 1997) entitled SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS, Barratt, et al., inventors. For a smart antenna system that has only one spatial channel per conventional channel, see co-owned U.S. patent application Ser. No. 08/729,390, filed on Oct. 11, 1996, entitled METHOD AND APPARATUS FOR DECISION DIRECTED DEMODULATION USING ANTENNA ARRAYS AND SPATIAL PROCESSING, Barratt et al., inventors.
With such systems, it therefore is important for maintenance personnel to know if one or more antennas is not functioning correctly for whatever reason. One such reason is that there is some faulty hardware in the antenna or the signal path of the output of the antenna. For example, if three out of four antennas in an array are working, the fourth antenna's failure will not usually cause an operational failure of the whole transceiver that would draw attention to the problem, so a failure of one antenna could go undetected for a long time. In the meantime, the loss of one or more antennas will degrade the performance of the base station and could result in lost revenues, decreased customer satisfaction, and even wasted maintenance efforts.
In addition to some of the hardware being faulty, it is also important to detect and quantify siting efficiency for multiple antenna systems. It is desirable for the antennas to be balanced. However, for example, there may be siting situations in which one of the antennas may be "hidden," i.e., it may be in an RF shadow and so won't receive signals very well. In such a situation, all the hardware may be working correctly, but the antenna system was not placed well. This is more likely in a micro-cellular environment (like the PHS or DECT system) than in a cellular system.
The term faulty antenna will thus be understood herein to mean an antenna in which either the hardware is faulty, or which produces a low signal level because of siting inefficiency.